The present invention relates to solid state lasers. More specifically, but without limitation thereto, the present invention relates to a solid state three level laser having a diffusion bonded composite gain element and an improved cooling system for attaining high output power levels.
In quasi-three level lasers, the laser transition terminates at a state which is near the ground state and is thermally populated. An example of such a laser is a neodymium doped yttrium aluminum garnet (Nd:YAG) laser operating at 946 nm. This emission involves a transition in neodymium from the .sup.4 F.sub.3/2 state to the highest of the ground .sup.4 I.sub.9/2 states. The energy of this terminal level is approximately 860 cm.sup.-1 above the lowest level, and about one percent of the neodymium atoms are thermally excited to this level at room temperature. This results in a ground state absorption at 946 nm and an increased pump energy threshold.
The physics of such laser systems are well known, and a number of publications have described the operation of these lasers both theoretically and experimentally. It is generally desirable to reduce optical path lengths in the laser gain element where the population inversion is insufficient to overcome the ground state loss. Further, to reduce the absorption loss, efficient cooling of the laser gain element is desirable to maintain the lowest practical operating temperature.
The most efficient operation of quasi-three level lasers at relatively low power has been demonstrated by end or longitudinal pumping a thin slab or disk of laser material. The material is often configured as an active mirror (i.e. one with gain) and the pump light is focused through an end face. A dielectric coating on the end face is designed to transmit pump light and also to serve as a highly reflective coating for the laser output. High pump power densities may be obtained using single emitter, high brightness laser diodes as a pump source, and a short active gain length helps to keep ground state absorption low. This results in a reasonably low pump threshold power and efficient laser performance.
With recent advances in laser diode array technology, it has now become possible to focus many tens of watts of pump light in the laser material in an area of only a few square mm. The simple techniques described above are not sufficient for operating such lasers at these higher power levels. Since the laser medium is typically cooled by conducting heat to a solid heat sink over the limited surface area of the short gain length, the heat conduction is less than optimum. Further, the faces of the laser medium where heat generation is greatest are typically exposed to the ambient atmosphere and are not directly cooled by the heat sink.
Despite efforts to improve cooling the laser gain element, a need continues to exist for a cooling technique that allows higher power levels to be attained for solid state three level lasers.